Insecticides, polychlorinated biphenyls (PCBs), chlorinated benzenes, chlorophenols, chloroquaiacols, chloroveratroles, chlorocatechols, as well as chlorinated aliphatics are contaminants whose toxicity can be decreased or eliminated by reductive dechlorination. This process involves the successive shedding of chlorine atoms under reduced anaerobic conditions and is usually biologically catalyzed.
Chlorinated ethenes (unsaturated C.sub.2 compounds), such as tetrachloroethylene (C.sub.2 Cl.sub.4) and trichloroethylene (C.sub.2 HCl.sub.3) are among the most frequently found contaminants in soils and aqueous environments. These contaminants, generally released from industrial and commercial sources, have become a ubiquitous presence in many ecosystems. Wide spread occurrence of chlorinated ethenes in the environment is of great concern due to their toxicity, carcinogenicity and persistence in the environment. Along with other low molecular-weight halogenated organic compounds, chlorinated ethenes are listed by the United States Environmental Agency as high priority pollutants.
Various factors complicate the removal of these contaminants from the environment. They are exceedingly volatile, highly mobile, denser than water, and generally found in the environment as mixtures of products with different degrees of chlorination. In addition, when these contaminants are present in soil, contamination is generally so extensive that excavation is impractical and cost prohibitive.
While removal of chloroethenes by pump and treat methods may offer a less costly alternative, these methods are just as impractical as excavation. In situ processes based on extraction, stripping, and/or adsorption on activated carbon can also remove chloroethene pollutants but these processes do not solve the problem of final disposal. Thus, much effort has been devoted to the development of practical and cost effective techniques for the removal of chlorinated contaminants from the environment, particularly from contaminated soils.
Bioremediation may offer a practical alternative for the removal of chlorinated contaminants from the environment. But for bioremediation techniques to be fully effective, significant progress must be made in enhancing the kinetics of the complex chemical processes underlying bioremediation.
For chlorinated ethenes in soil, recalcitrance in the presence of oxygen can be alleviated during co-metabolic degradation in aerobic environments by delivering natural gas or methane to the subsurface. In this context, methane serves as a carbon rich energy source for methanotrophic bacteria which also coincidentally metabolize some chlorinated contaminants. Under anaerobic conditions, all chlorinated ethenes can be completely transformed into benign end products through sequential reductive dechlorination. In this biologically catalyzed process, chlorinated pollutants act as electron acceptors, the chloride moiety is removed from molecules and replaced by hydrogen. The availability of a suitable electron donor for this process is one of the key rate limiting elements.
Transformation of perchloroethylene (PCE) proceeds by sequential reductive dechlorination to trichloroethylene (TCE), dichloroethylenes (DCEs), vinyl chloride (VC), and ethylene (ETH). ETH is a commonly occurring plant hormone that has not been associated with any long-term toxicological problems.
FIG. 1 is a schematic diagram of the conversion pathway from PCE to ETH. The diagram shows two of the three possible DCE isomers. 1,1-DCE is the less significant isomer and is not shown. Cis-1,2-DCE predominates over trans-1,2-DCE.
The rates of reductive dechlorination reactions are a function of the chemical structure of the chlorinated compound. Highly chlorinated, and consequently highly oxidized, ethenes are rapidly dechlorinated via this process, while less substituted ethenes are more resistant to reduction. Consequently the rate-limiting step in reductive dechlorination is the conversion of VC to ETH. ETH is considered an end product although further degradation of ETH to CO.sub.2 may occur. Such degradation is thought to be hindered by the observed recalcitrance of ETH attributed to its role as a potent selective inhibitor of methanogenesis.
Sustaining reductive dechlorination is highly dependent upon the availability of an electron donor (reductant) in the contaminated environment. Some of the compounds that have been found to support the reductive transformation of PCE are glucose, acetate, formate, methanol, lactate, propionate, crotonate, butyrate, ethanol, and other compounds such as toluene and dichloromethane. Higher dechlorinating activities were observed when organic substrates that contain more reducing power during anaerobic digestion (e.g. formate, glucose, lactate) were used. Also, electron donors that produce hydrogen more slowly give a selective advantage to organisms that dechlorinate chlorocarbons over those that generate methane. Thus, It would be highly desirable to provide a method that uses hydrogen as the direct electron donor in the removal of chlorinated compounds through the process of reductive dechlorination.
FIG. 2 is a graph showing the influence on PCE degradation when hydrogen is added to the system. The graph shows that PCE degradation is substantially higher in cultures supplied with hydrogen compared to cultures not supplied with hydrogen. The slow degradation of PCE in the absence of added hydrogen is probably supported by the yeast extract initially present in the basal medium.
At the mechanistic level, reductive dechlorination is a biologically catalyzed chemical process. Reductive dechlorination occurs readily in a variety of complex anaerobic communities without acclimation, or with relatively short acclimation times (less than 1 month). This suggests that the catalyst is non-specific and continuously present in the natural community. Transition metal complexes (porphyrins), common to many enzymes, have been found to be involved in these processes. For example, vitamin B.sub.12 (Co-containing porphyrin) which is often found in bacteria, and coenzyme F.sub.430 (Ni-containing porphyrin), found only in methanogenic bacteria, have the capacity to mediate the eight-electron sequential reduction of PCE to ETH.
The roles played by major classes of microorganisms inhabiting mixed cultures capable of dechlorinating synthetic compounds are still not exactly known. The hypothesis that the dechlorinating organisms are hydrogen utilizers that are nutritionally dependent on other organisms in the more diverse system has been examined. It is suggested that methanogens play a key role in the process. For example, degradation of TCE was completely stopped when bromoethane sulfonate (a selective inhibitor of methyl-coenzyme-M reductase which catalyzes the final step in methanogenesis) was added to mixed cultures. On the other hand sustained dechlorination in the presence of vancomycin which inhibits acetogenesis suggests that acetogens are probably not the dechlorinators.
Direct dechlorinators that utilize chlorinated ethenes as electron acceptors in an energy-conserving, growth-coupled metabolism termed dehalospiration may also contribute to the process of reductive dechlorination. These microorganisms must compete for available hydrogen with hydrogenotrophic methanogens and sulfate reducers and because of the relatively high energy available from reductive dechlorination, it is reasonable to suspect that they may out-compete methanogens at very low hydrogen levels. Competition for hydrogen is thus a very important aspect of the reductive dechlorination process. The partitioning of hydrogen flows among the various competitors is a function of the hydrogen concentration, which itself depends on the rates of hydrogen production and utilization.
Compounds such as lactate or ethanol that can be rapidly fermented to acetate, producing high short-lived peaks of hydrogen, do not favor dechlorination as well as would more persistent, slowly fermented substrates such as benzoate or propionate. Thus it would be highly desirable to design new techniques and methods that would allow slow H.sub.2 release, closely matching the rate of metabolic uptake by the dechlorinators. Such techniques and methods would allow a significant increase in the rate of in situ degradation of chlorinated pollutants.
Chlorinated solvents, such as trichloroethylene and perchloroethylene can be degraded by reactions with granular iron. This abiotic reductive dehalogenation is thought to proceed with pseudo-first order kinetics. Although details of the chemical mechanisms involved in this process are yet to be fully elucidated, the process is thought to involve the simultaneous oxidative corrosion of the reactive iron metal by both water and the chlorinated organic compounds. The two half-reactions involving iron and TCE can be shown as: EQU Fe.degree..fwdarw.Fe.sup.2+ +2e.sup.- (1) EQU C.sub.2 HCl.sub.3 +3H.sup.+ +6e.sup.-.fwdarw.C.sub.2 H.sub.4 +3Cl.sup.- (2)
These are accompanied by the decomposition of water and subsequent formation of hydrogen gas: EQU 2H.sub.2 O+2e.sup.-.fwdarw.H.sub.2 (g)+2OH.sup.- (3)
This chemical degradation process is assisted by a parallel biological route where the growth of methanogenic bacteria is stimulated by the produced hydrogen, and these organisms enzymatically dechlorinating additional contaminants.
Bimetallic preparation of iron with a small amount of Pd (0.05% by weight) can substantially enhance the dechlorination rate of volatile organic compounds. Similarly, adding a nickel coating to iron particles greatly enhances the dechlorination kinetics of many chlorinated VOCs, typically by an order of magnitude over reduction by iron alone. Both palladium and nickel enhance the generation of hydrogen gas, as described below. As suggested by equation (2), TCE degrades spontaneously in the presence of both water and iron, requiring no additives or application of energy, and the products of the chlorinated compounds are chloride and nontoxic hydrocarbons. The method offers a "passive treatment method" which can produce a substantial cost reduction in anticipated operation and maintenance expenses for a remediation project.
The very low levels at which toxicants are present in ground water is a factor in designing suitable treatment systems. Although the quantity of reductant that meets the stoichiometric requirements is small, a large excess of iron is required to provide a reactive surface area large enough for the contaminants to adsorb on the surface and the reaction to occur. Lengthy "flow-through" times (i.e., residence times) are also needed. Usually, treatment zones are created by forming a fixed bed reactive barrier, of either a continuous wall or funnel-and-gate type system. In either case, iron is placed deep enough to intercept the saturated thickness of the plume in a contaminated zone. Sometimes above ground reactors are used. The rates of dehalogenation vary widely for the various chlorinated solvents of interest, and when the design of a treatment method includes a mixture, the design of a barrier is determined by the least reactive constituent. Rate constants normalized to iron surface area have been used as the basis for barrier design and size considerations. For example, a model for calculating the amount of iron needed for 1000 fold decrease in contaminants has been developed. With the unit cost of granulated iron at approximately $450/ton, the amount of iron required represents a substantial cost. Even modest sized treatment barriers, 2 ft (w).times.100 ft (l).times.50 ft (d), that would be capable of treating ground water flow velocity of no greater than 1 ft/day, would cost nearly $1 million in granulated iron alone. Thus, potential operating and maintenance cost advantages of reactive metal barriers are in some ways offset by high installation costs. Furthermore, the method is complicated by the need to ensure that the ground water is directed through the barrier and to avoid the possibility of flow around the edges. Another limitation of these techniques is related to the clogging that may occur with long term use of zero-valent iron, thus it is expected that the surface will require regeneration, an issue that has been given little attention at the present time.
U.S. Pat. No. 5,510,201 discloses a hydrogen producing system that utilizes corrosion of iron by water. The disclosed system invokes the potential benefit for regenerating iron subsequent to corrosion. The invoked methods for regenerating iron are, however, limited to chemical treatment by providing reforming fuels to the corroded iron. Such treatments necessitate the collection of the corroded iron for delivery to central plants where it would be regenerated. Such regeneration techniques are impractical in the context of contaminated environment bioremediation.
U.S. Pat. No. 2,623,812 discloses a hydrogen production system based on the use of metal particles combining iron and magnesium. The combination is obtained by mixing powders of iron and magnesium. While the disclosed combinations yield some enhancement in the rate of hydrogen production, the maximum improvement that can be obtained through this technique is drastically limited by the method of preparation of the metallic combination. More intimate combination of the metals is particularly desirable in the context of bioremediation of contaminated soils which require the production of significant quantities of hydrogen for extended periods of time.
In U.S. Pat. No. 3,942,511, Black et al. recognized the need for higher intimacy between iron and magnesium for a combination of the metals to yield higher rates of hydrogen production. Their proposed method for achieving higher intimacy between the metals is based on improved contact between the metal particles, achieved through compressing of the particles. Black et al. also disclose the need in their method for adding salts to the metal combination in order to achieve a higher contact between the metals.
U.S. Pat. No. 4,264,362 also discloses a mechanically based metal mixing technique for the formation of bimetallic particles with higher dispersion of smaller particles of one metal in larger particles of another metal.
In U.S. Pat. No. 3,957,483, Suzuki discloses a method for enhancing hydrogen production by corroding iron. His method is also mechanically based. Iron particles are mechanically attached to magnesium particles.
While the above described attempts seem to recognize the need for a higher degree of intimacy in combining iron and magnesium to increase the rates of hydrogen production, all the disclosed techniques are limited to one version or another of mechanically mixing the metals. The degree of intimacy obtainable through any mechanical mixing method is inherently limited by the size of the smallest particle obtainable by mechanically disintegrating a larger metal particle. Thus, in hydrogen production by corrosion of a metallic particle, multi-metallic particles with degrees of intimacy beyond what can be achieved through mechanical mixing are highly desirable. Such particles are particularly desirable in applications that necessitate the production of high quantities of hydrogen for long periods of time such as bioremediation of contaminated soils.